Power MOSFETs are the preferred switching devices for notebook computers and other portable electronic devices, and they are also widely used for switching currents in the automotive industry. In a common form of MOSFET, the gate electrode is formed in a trench that extends downward from the surface of the chip, and current flows primarily in a vertical direction between a source region on one surface of the chip and a drain region on the other surface of the chip. The source region is normally shown on the top surface of the chip and the drain region is shown on the bottom surface of the chip, although this orientation is arbitrary. The trench is lined with a dielectric layer (typically silicon dioxide), and a channel is formed in a body region adjacent a wall of the trench. When the gate is properly biased (positive in an enhancement-mode N-channel device, negative in an enhancement-mode P-channel device) the channel becomes inverted and allows current to flow between the source and the drain. In depletion-mode devices the MOSFET is normally turned on and is turned off by a proper gate bias (negative in a depletion-mode N-channel device, positive in a depletion-mode P-channel device).
Two of the principal performance criteria of a power MOSFET are its on-resistance (Rdson) and its avalanche breakdown voltage VB. Rdson is a measure of the resistance through the MOSFET when it is turned on and VB is a measure of its ability to block a reverse voltage. Another important performance criterion is the capacitance between the gate and drain (Cgd), which determines the MOSFET's ability to switch current quickly and operate at high frequencies. In normal trench-gated MOSFETs the gate-to-drain capacitance is measured across the gate oxide layer at the bottom of the trench, which separates the gate electrode from the drain.
It is known to increase the breakdown voltage VB by including a “drift region” between the body and the drain of the device. The drift region is a relatively lightly-doped region of the same conductivity type as the drain. While the inclusion of a drift region in the device tends to improve VB, it also tends to increase Rdson, since the drift region represents a relatively lightly-doped region that the current must traverse when the MOSFET is turned on.
Various techniques have been proposed for reducing Cgd. One proposal, suggested in U.S. Pat. No. 4,914,058 to Blanchard, is to increase the thickness of the gate oxide layer at the bottom of the trench. This technique is illustrated by MOSFET 10, shown in the cross-sectional view of FIG. 1. MOSFET 10 is formed in an epitaxial (epi) layer 102 that is grown on an N+ substrate 100. A trench 104 extends through epi layer 102 and into N+ substrate 100. Since MOSFET 10 is an N-channel device, epi layer 102 is generally doped with an N-type impurity such as phosphorus. Epi layer 102 also includes an N+ source region 106 and a P body 108, both of which are contacted by a metal layer 115. The background N-type doping of epi layer 102 is found in an N-drift region 110. N+ substrate 100 and N-drift region 110 represent the drain of MOSFET 10.
The sidewalls of trench 104 are lined with a gate oxide layer 112, and trench 104 is filled with a gate electrode 114, which is typically made of polycrystalline silicon (polysilicon) that is doped heavily to make it conductive. At the bottom of trench 104 is a thick oxide layer 116 that serves to reduce the capacitance between the polysilicon gate 114 and the drain (N+ substrate 100 and N-drift region 110). The Rdson of MOSFET 10 can be reduced somewhat by providing a graded doping concentration in N-drift region 100, decreasing gradually in the direction from N+ substrate 100 to P body 108, but nonetheless Rdson is still not below the silicon limit which is the minimum Rdson for a given breakdown voltage BV. The silicon limit is defined by the equation, Rdson=5.93×10−9*BV2.5.
A two-step etching process was described for fabricating this device. First, a gate trench mask was used to form trenches of a desired width and depth. A thin gate oxide was grown on the walls and floor of trench 104, and a nitride layer was deposited over the gate oxide layer. A directional etching process (e.g., reactive ion etching (RIE)) was used to remove the nitride and gate oxide from the floor of the trench, and a second trench was etched through the floor of the trench reaching to the N+ substrate 100. Thick oxide layer 116 was formed in the second trench. The other process steps were similar to those customarily employed in trench MOSFET fabrication.
U.S. Pat. No. 5,637,898 to Baliga describes a process that uses a single-trench etch and an oxidation that creates a thick bottom oxide. Polysilicon is subsequently deposited and etched, leaving a recessed polysilicon layer at the bottom of the trench. The sidewall oxide is then etched away, and a new gate oxide layer is grown, followed by a selective RIE process to remove the oxide layer formed on top of the recessed polysilicon layer. Polysilicon is then deposited to form the desired thin-thick gate oxide layer realized by Blanchard in the two-step etch process described above. Baliga also uses a graded doping profile in the drift region to reduce the on-resistance.
U.S. Pat. No. 5,998,833 to Baliga teaches another type of trench MOSFET. The trench contains an upper gate electrode, which is generally aligned with the source and base regions, and a lower source electrode, which is generally aligned with the drift region. Again, the drift region is linearly graded and decreases in a direction from the drain region to the surface of the silicon. However, the bottom of the upper gate electrode is aligned with the junction between the P-base region and the N-drift region. This requires that both the polysilicon layer that is deposited to form the lower source electrode be etched and the oxide layer separating the upper and lower electrodes be formed to a high degree of accuracy. If, for example, the lower source is not etched deeply enough, or if the oxide layer separating the upper and lower electrodes is grown too thick, the bottom of the gate electrode will be located above the junction between the base and drift regions. As a result, the upper gate electrode will not invert the entire channel and the device will not turn on. U.S. Pat. No. 6,388,286 to Baliga describes a trench structure that has similar problems.
Recently, an article by X. Yang et al. (“Tunable Oxide-Bypassed Trench Gate MOSFET: Breaking the Ideal Superjunction MOSFET Performance Line at Equal Column Width,” IEEE Electron Device Letters, Vol. 24, No. 11, pp. 704-706, 2003) described a trench oxide bypass structure that had very low Rdson. Drawing on a concept previously proposed by Y. C. Liang et al. (“Tunable oxide-bypassed VDMOS (OBVDMOS): Breaking the silicon limit for the second generation,” Proc. IEEE/ISPSD, pp. 201-204, 2002), this article reported the successful fabrication of a TOB-UMOS device having a 79 V rating. The device reportedly broke the ideal superjunction MOSFET performance line at an equal column width of 3.5 μm and potentially the ideal silicon limit as well.
Nonetheless, there is a clear need for a new type of MOSFET whose on-resistance is lower than what can be achieved following conventional MOSFET structures.